The present invention relates to a method and apparatus for applying a coating to a web of material and more particularly to a low profile coating unit for use in hot dip galvanizing of a continuously moving strip of steel of indefinite length.
In the manufacture of galvanized steel by the hot dip process, a strip of steel is drawn through a molten bath of zinc or zinc alloy and drawn upwardly where it passes between a pair of air knives which blow air, steam or other fluid against the opposite sides of the sheet prior to the zinc or zinc alloy coating material becoming fully set in order to blow excess amounts of such coating material from the surface of the sheet and, by variations in pressure, spacing between the surface of the strip and the air knives and the time interval between exiting the bath and entry between the air knives, control the thickness of the galvanized coating on the strip of material. For example, ASTM specification 525 published by the American Society for Testing and Materials, Philadelphia, PA, provides for coating designations for regular types ranging from G 235 which has an optimum coating of 2.35 ounces per square feet of sheet (triple-spot test) to G 30 which has 0.30 ounces per square feet of sheet and even down to G 01 which has no minimum specified for the coating weight. Thus, for different applications different thickness of coating are desired. Additionally, for different applications of the finished product, it is desirable to have galvanized steel strips of different thicknesses. Thus, the thickness of the metal desired to be coated may be as thin as 0.010 inches or as thick as 0.250 inches.
The prior art shows many different types of apparatus and methods for processing the steel strip as it is drawn upwardly from the bath of molten zinc or other coating material including a wide variety of air knives of various designs and mechanisms for supporting and adjusting such air knives.
U.S. Pat. No. 3,783,824 is directed to an elevator construction for coating control equipment. The equipment disclosed in such patent platform shows means for supporting the air knives or air nozzles with the platforms supported by linkage means which has the capability of imparting straight line vertical motion and straight line horizontal motion to the platform means, and also provide means for moving the nozzle assemblies in a lateral direction relative to the strip.
U.S. Pat. No. 3,681,118 discloses a method of removing molten metal coatings by low pressure gas streams. A pair of nozzles are swingably mounted at opposite ends of sliders so that the gas ejection angle of the nozzles with respect to the opposite faces of the coated strip can be adjusted within the range of 3.degree. to 45.degree.. The mechanism permits the nozzles to be moved toward and away from the strip face independently of each other by supplying fluid under pressure to hydraulic cylinders. It also provides for a wedge mechanism for close adjustment of the spacing between the nozzles and the steel strip.
U.S. Pat. No. 4,719,129 discloses a plurality of rotatably mounted jet finishing knives mounted to an assembly which can be raised or lowered by a crank mechanism. The coating thickness of a strip can be varied simply by rotating the knives such that a knife having a different orifice height from the previous knife impinges pressurized gas against the web of material. The knives are enclosed in a protective atmosphere.
A number of prior art patents show specific designs of air knives. For example, U.S. Pat. No. 3,314,163 discloses a nozzle which may be opened for cleaning, which provides uniform jet across the width of the steel strip that is free of turbulence and which may have the gap and the angle of impingement adjusted independently of one another. Additionally, it discloses the desirability of eliminating or at least minimizing turbulence present in the air in order to provide a smooth flow of air against the surface of the strip being treated.
U.S. Pat. No. 3,526,204 discloses a knife which increases wiping action at the strip edges and which is adjustable for various strip widths.
U.S. Pat. No. 3,841,557 discloses nozzles of air knives in which the width of the gas outlet slot in each nozzle is controlled by thermally expanding at least a portion of the walls of the slot.
U.S. Pat. No. 3,977,359 discloses a composite air knife having shutters at both ends of the nozzle to adjust the effective length of the slot formed by the lips of such nozzle.
U.S. Pat. No. 4,041,895 discloses an air knife having internal devices to provide directionally controlled laminar flow that has a pressure profile which varies along the length of the knife.
U.S. Pat. No. 4,106,429 discloses an air knife with a slideable lip to permit adjustment of the opening.
U.S. Pat. No. 4,198,922 is directed to a gas barrier assembly for controlling coating thickness on a strip of steel after it leaves a bath of molten zinc. It discloses as prior art the feature of directing air against the steel strip from curved lip orifices as well as a new feature of providing support means having readily releasable connecting means which permits the gas orifice segment to be removed and replaced without removing the frame from the coating line.
Other patents which disclose various types of arrangements for opening or disconnecting for cleaning purposes include U.S. Pat. Nos. 4,359,964; 4,513,915 and 4,697,542.
The prior art devices, while suitable for many applications are massive in size, complex in their construction and lack the flexibility to handle a full range of steel strip thicknesses and coating gauges on a single line. Additionally, the prior art air knives are themselves complex and difficult to maintain.